Erosion-Corrosion Failures in Wellhead Chokes

Surface morphology mapping was performed in two chokes for oil wellheads which damage patterns differs one from the other and attempts were done to relate the observed morphologies with the material degradation mechanisms.

In one of the chokes, the damage concentrates around the annular gap between the reducer and the choke-case. Near the thread jointing both components, severe circumferential channeling develops, leading to decoupling. One of the channeling branches crosses the case wall producing leakage, this trajectory being enhanced by defects in the girth weld, inappropriately placed near the thread. Downstream, the surface groves become shallower and bear microscopic evidences of CO₂ induced corrosion: pits and carbonate containing deposits. Beyond this zone, corrosion turns into the governing damage mechanisms, as shown by dense and almost uniformly distributed pits.

In the second case, the most intense damage appears downstream the reducer, being induced by the corrosive main flow that carries about 45 % formation-water with a CO₂ content of 30 to 110  psia. The balance between erosion and corrosion varies with the distance from the reducer outlet. Dense coalescing pits characterize the corrosion controlled zone, which is coincident with a girth weld; large and deep cavities were observed in the heat affected zone of this weld, one of them crossing the wall and producing fluid leakage. Smooth channels, eventually bearing corrosion deposits brought by the flow, were observed in the erosion controlled zone.